1. Field of the Invention
The present invention relates to a method of correcting curl of a sheet and a recording apparatus for carrying out the method of correcting curl of the sheet.
2. Description of the Related Art
In recording apparatuses such as a printer, a facsimile, and a copying machine, an image is recorded on a sheet (a recording medium) by a recording head in accordance with image information. The recording apparatuses can be classified into various types depending on a recording method performed by the recording head. One type of the recording apparatuses is, e.g., an ink jet recording apparatus in which recording is performed by discharging ink to the recording medium through discharge ports of the recording head. In the ink jet recording apparatus, recording is performed by causing ink droplets to fly to a recording surface. Therefore, if the recording medium is too close to the recording head, the recording medium and the recording head would come into contact with each other, thus generating an ink stain and smear. If they are far apart from each other, the ink droplets would not land on correct positions on the recording surface. To cope with those drawbacks, it is required to properly manage the distance between the recording head and the recording medium (hereinafter also referred to as the “head-to-paper distance”). In order to maintain the proper head-to-paper distance, it is also often required to ensure flatness of the recording medium.
One proposal for maintaining the proper head-to-paper distance is to convey a recording medium to a recording section after correcting curl of the recording medium in advance. The curl is generally corrected by curving the recording medium in a direction reversal to the curling direction of the recording medium. For example, the curl is corrected by a method of pressing a decurl roller against the recording medium, which is conveyed in a state gripped by a pair of paper feed rollers and a pair of conveying rollers, such that a curvature is given to the recording medium in the direction reversed from the curling direction between both the roller pairs. However, the known curl correction method has the issue of a curvature to a leading end portion of the recording medium and the curl is apt to remain in the leading end portion of the recording medium.
Further, the leading end portion of the recording medium tends to cause particular deformations, such as folding and skewing, and a curl condition is apt to disorder in the leading end portion of the recording medium. From the viewpoint of managing the head-to-paper distance, therefore, the leading end portion of the recording medium requires dedicated treatment differing from that required in the other portion of the recording medium. For example, when the recording medium is conveyed to the recording section, the recording medium is desirably curled downward so that the leading edge of the recording medium will not contact the recording head. On the other hand, if the recording medium is entirely curled downward, the recording surface is caused to convex toward the recording head and is more likely to contact the recording head. Further, in some cases, dedicated curl correction is required for only the leading end portion of the recording medium such that the leading end portion can be easily led into the nip between a pair of conveying rollers in the recording section.
In general, the curl of the recording medium is corrected by curving the recording medium in the reversed direction while tension is applied to the recording medium. However, because the leading end portion of the recording medium is gripped after being passed through the nip between a pair of gripping rollers, it is difficult to strongly curve the leading end portion of the recording medium and to correct the curl in the leading end portion. For that reason, using a special curl correction unit adapted for the leading end portion of the recording medium is proposed. For example, Japanese Patent Laid-Open No. 08-026564 proposes a method of temporarily guiding only the leading end portion of the recording medium to a separate mechanism unit and curving the leading end portion by using a special mechanism. Further, Japanese Patent Laid-Open No. 2006-168948 proposes a method of enhancing the curving of the recording medium to correct the curl by repeating operations of feeding the recording medium forward and backward plural times.
However, the method proposed in Japanese Patent Laid-Open No. 08-026564 requires addition of the special mechanism to correct the curl in the leading end portion of the recording medium, and has the problem that an apparatus is complicated and its size tends to increase. Further, with the method proposed in Japanese Patent Laid-Open No. 2006-168948, curl correction power is adjusted by curving the leading end portion of the recording medium plural times, but problems arise in that there is a limitation in curving the leading end portion to give it a curvature, the operation time is prolonged, and the curving direction is restricted.